MINOR: [C++][Parquet][Docs] Increase chunk_size in docs (#40705)

### Rationale for this change

Is there a reason we're using a low value here. All other examples use `128*1024` or `64*1024`.

I was stumped by this as I used it without really reading about the parameter and spent a day figuring out why my parquet writes were so slow.

### What changes are included in this PR?

Increase `chunk_size` to `64*1024`

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.

Authored-by: Shadab Zafar <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

Author: dufferzafar

cpp/examples/arrow/parquet_read_write.cc

@@ -120,7 +120,7 @@ arrow::Status WriteFullFile(std::string path_to_file) {
 
   ARROW_RETURN_NOT_OK(parquet::arrow::WriteTable(*table.get(),
                                                  arrow::default_memory_pool(), outfile,
-                                                 /*chunk_size=*/3, props, arrow_props));
+                                                 /*chunk_size=*/64*1024, props, arrow_props));
   return arrow::Status::OK();
 }